A figure space or numeric space is a typographic unit equal to the size of a single numerical digit. Its size can fluctuate somewhat depending on which font is being used. This is the preferred space to use in numbers. It has the same width as a digit and keeps the number together for the purpose of line breaking.
In Unicode it is assigned U+2007 FIGURE SPACE. Its HTML character entity reference is  .
Baudot code may include a figure space. It is character 23 on the Hughes telegraph typewheel.
WORD JOINER should be used if the intent is to merely prevent a line break